Local Electron Spin-State Engineering at Fe Sites for Highly Reversible Sodium-Ion Batteries

Developing robust cathode materials for sodium-ion batteries (SIBs) requires the simultaneous mitigation of sluggish reaction kinetics and inadequate structural stability during cycling. Herein, a distinctive carbon-coated sodium iron sulfide composite (Na₆FeS₄/C) is developed via a rotary evaporation-carbothermal reduction strategy, and spin-state engineering is first proposed to induce electronic spin polarization at Fe sites. The short and nearly identical axial (2.356 Å) and equatorial (2.355 Å) bond lengths in Na₆FeS₄ indicate a thermodynamically stable crystal structure. Furthermore, the d-p orbital hybridization between Fe 3d and S 3p orbitals in the [FeS₄]^6- tetrahedron can accelerate electron delocalization and optimize charge distribution, thus improving charge transfer dynamics and capacity output. More importantly, S coordination induces the electron transition of Fe²⁺ from the d_z ² orbital to the d_xz orbital to form high-spin (HS, e³t₂ ³) Fe atoms, which could generate more unpaired electrons on the 3d orbitals and enhance the electrochemical activity of Fe²⁺ sites. Based on these merits, the well-designed Na₆FeS₄/C composite cathodes exhibit exceptional Na storage performance, delivering a high initial reversible capacity (419.8 mAh g^-1 at 0.1 C), excellent rate performance (192.2 mAh g^-1 at 2 C), and superior long-term cycling stability (83.3% capacity retention rate over 400 cycles at 1 C).